As an example of error control technology used in mobile communication system, there is a type-2 hybrid ARQ (Automatic Repeat reQuest) method, (hereinafter, it is referred to as “type-2 hybrid ARQ method”). Such a type-2 hybrid ARQ method uses the turbo code and is referred to as an IR (Incremental Redundancy) method.
In this method, turbocoding is performed intransmission apparatus as shown in FIG. 1, among turbo encoded signals, an information bit (also called “systematic bit”) (S) is first transmitted, then an error detection is carried out in the reception apparatus. When an error is detected, a NACK (Negative ACKnowlegement) signal is returned back to transmission apparatus from reception apparatus. In this case, transmission apparatus transmits parity bit 1 (P1) (redundancy bit) of FEC (Forward Error Correction) for use in an error correction, and reception apparatus carries out turbo decoding using information bit and parity bit 1. Moreover, when an error was detected, in response to the NACK signal from reception apparatus, transmission apparatus transmits parity bit 2 (P2) of FEC for use in an identical error correction, and reception apparatus carries out turbo decoding using information bit, parity bit 1, and parity bit 2. In addition, when there is no error, an ACK (ACKnowlegement) signal is returned back and the next data is requested.
However, the aforementioned method includes the following problem.
In turbo code, the quality of an information bit in the reception signal reflects a large influence in the quality of signal after decoding. In other words, in the case when the quality of information bit is bad (for instance, low SNR), it may be difficult to carry out good decoding even when the parity bit quality is high, hence, it may be also difficult to obtain a decoded signal with high quality. Therefore, in type-2 hybrid ARQ method, information bit is first transmitted, and in the case when the first transmitted information bit is received with low SNR because the quality is deteriorated due to fading etc. as parity bit is transmitted in the case of retransmission, the quality after combining of a large number of retransmitted parity bits may not be improved and a useless retransmission may be continued.
In view of the aforementioned problem, it is an object of the present invention to provide a data communication apparatus and data communication method based on type-2 hybrid ARQ method which can improve quality of transmission data while avoiding the increase of power consumption.
As a method of transmitting a high speed packet with good efficiency, HSDPA (High Speed Down-link Packet Access) is proposed in 3GPP (3rd Generation Partnership Project). In such a method, the transmission rate is updated by changing channel codec, spreading factor, multiplexing (multiplex value), or modulation according to channel environment, and the average throughput is improved using such a method.
When an adaptation modulation of HSDPA is carried out, it is necessary for transmission apparatus (radio base station) to transmit an MCS (Modulation and Coding Scheme) to reception apparatus (communication terminal). MCS is information related to transmission packet data such as the modulation method (QAM (Quadrature Amplitude Modulation), QPSK (Quadrature Phase Shift Keying), 8PSK (8 Phase Shift Keying)), etc., or the coding rate (R=½, ⅓), etc. Reception apparatus demodulates data of packet channel using such MCS information, and carries out a decoding processing.
However, there is a problem that the capacity of downlink channel decreases since the transmission side (radio base station) transmits the MCS information. In addition, there is another problem related to transmission of MCS information within a cell that is an increase of inter-cell interference is provided.